Magnetostriction is the phenomena whereby a material changed shape (i.e., dimensions) in the presence of an external magnetic field. This effect is brought about by the reordering of the magnetic dipoles within the material. Since the atoms in a magnetostrictive material are not, for all practical purposes, perfectly spherical (they're shaped more like tiny ellipsoids) the reordering of the dipoles causes an elongation (or contraction depending on the mode of reorientation) of the lattice which leads to a macroscopic shape change in the material. There is a “reverse magnetostrictive effect”, called the Villari effect: When an external stress is applied to a magnetostrictive material, a strain develops within the material which induces a surrounding magnetic field. Known magnetostrictive materials include alloys of iron (Fe), Nickel (Ni), cobalt (Co), yttrium (Y), gadolinium (Gd), terbium (Tb), dysprosium (Dy), and so on.
The so-called magneto-elastic effect is a phenomenon exhibited by ferromagnetic substances. It refers to the interdependence of the state of magnetization and the amount of mechanical strain present in the material and manifests as magnetostriction, volume change upon magnetization and, inversely, changes in the state of magnetization upon application of stress. When a sample of magnetostrictive material is subjected to an applied small time-varying (AC) magnetic field superimposed on a much larger direct-current (DC) magnetic field, the magnetic energy is translated into elastic energy and the sample starts vibrating.
The mechanical vibrations are most pronounced as the frequency of the applied AC field gets closer to the characteristic resonant frequency f0 of the magnetostrictive sample and a voltage peak for emissions radiating from the sample can be registered by a pick-up coil in proximity thereto. This pronounced conversion from magnetic to elastic energy holds true at harmonics of resonant frequency f0 This condition is known as magneto-elastic resonance. One example of magnetostriction is the “transformer hum” we hear when a transformer core “pulsates” upon the application of a 60 Hz magnetic field, i.e., the ‘hum’ is the emission of acoustic energy that generates sound.
When measuring mechanical stress, strain or force, it is known to make use of measuring sensors based on magneto-elastic material. Magneto-elastic material has the advantage that it enables contactless signal transmission from a magneto-elastic sensor element to an electronic unit for evaluation of the signal from the sensor element. The relative permeability of a magneto-elastic element depends on the mechanical stress to which the element is subjected, e.g. by a strain within the surface on which it is mounted. During this process, signal scanning can be achieved with the aid of a coil system, the inductance of which is influenced by the permeability of the magneto-elastic element.
An important application area for magneto-elastic sensors is torque measurement on rotating shafts. A primary technique is currently utilized to measure strain and involves the use of torque sensing devices based on the torque-induced changes in the magnetization load-bearing element of the shaft. One can thus measure a flux decrease or increase using this technology. This methodology, however, is subject to high hysteresis results, and is also sensitive to ambient interference and costs a great deal with respect to other sensor technologies, such as, for example Surface Acoustic Wave (SAW) applications.
It is therefore believed that a solution to the aforementioned problems involves the implementation of an improved torque sensor device based on the use of magneto-elastic components. Such an improved sensor device is disclosed in greater detail herein.